Gamma spectroscopy with an SCA

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ChristofferBraestrup
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Joined: Sat Apr 11, 2020 9:47 am
Real name: Christoffer Braestrup

Re: Gamma spectroscopy with an SCA

Post by ChristofferBraestrup »

I try avoiding 75 ohm BNC cables, since the low-end ones are a whole dimension worse than the low-end 58 ohm ones, and the fit of the connectors are so bad I'm worrying about damaging the instrument connectors. If I'm in doubt, I use a 'sacrificial' T-adaptor of decent quality inbetween.

I've done some troubleshooting on my SCA. Here's what I found:

Tennelec TC441 (mid-1970's vintage):

Delta-E 0-10V range:

panel set: actual window size:
0 mV 3.84-4.0 7= 230 mV
100 mV 3.76-4.07 = 310 mV
200 mV 3.65-4.07 = 420 mV
300 mV 3.54-4.07 = 530 mV
400 mV 3.45-4.07 = 620 mV

Delta-E 0-1V range:

panel set: actual window size:
0 mV 0 - 0 = 0 mV
100 mV 3.96-4.07 = 110 mV
200 mV 3.86-4.07 = 210 mV
300 mV 3.75-4.07 = 320 mV
400 mV 3.65-4.07 = 420 mV
500 mV 3.55-4.07 = 520 mV


Keep in mind, voltages are set by dial alone, so the lat two significant digits are dubious.

This looks as if the 0-1V range is spot on, and the 0-10V one is offset 200 mV. Strange, but I guess it isn't critical, as long as one knows. Maybe someone modified it to suit a certain experiment ( a fixed window will always be more precise than variable) or maybe it's just supposed to be like that (I don't have the manual).

Also an interesting note is that the upper window limit is unchanged, it's really a -delta-E window, since it moves the window below E. Not sure if this is standard.



A neat trick I found with the Ortec 771 timer: connecting its STOP to RESET connection on the back of the unit, the INTERVAL connector will give a short positive 5V pulse every time the preset is met.
I used this as a digital pulse generator for my experiment, feeding it to my preamp, to my spec. amp, to a gate stretcher and to the SCA.



The window size was determined by feeding this fixed height (ca. 4V) signal into the SCA, then setting some window size, then moving the window up above the voltage and then down slightly, noting the voltage at which it appeared, then disappeared.

I must say, the absolute best tool for troubleshooting NIM modules is having multiple modules that does the same, or are identical. If two modules behave the same it's less likely a fault and more likely the user's understanding/something else in the signal chain that's causing the odd result.
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Richard Hull
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Joined: Fri Jun 15, 2001 9:44 am
Real name: Richard Hull

Re: Gamma spectroscopy with an SCA

Post by Richard Hull »

I have several modules that have been purchased at hamfests that are untested. I work on them easily. I took a 1 slot worthless module, gutted it and ran a straight through NIM plug on a cable. This allows power-up from a regulation bin and the module can lay flat on a table in front of the bin with full access to scope and meters. It can be flipped over as needed to access both sides for repair and soldering. Re-calibration is a snap.

I often find out much about the module that is interesting during test. With no manual, I do a quick little write up of how its functions on the more complex modules for later reference. I guess I have owned, used, repaired and sold a total of over 100 NIM modules and about 10 Bins. I might have about 40 modules left floating around. Most all were purchased from Don Orie of O.E. technologies when that wonderful edifice existed back in the 1990's. I have manufactured from scratch, using worthless modules, about 5 different custom modules.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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